1. Field of the Invention
The present invention generally relates to a diagnostic tool that can measure voltage under controlled conditions and vary a point load in order to determine the capacity of a given circuit to operate under typical or extreme conditions rather than ideal conditions.
2. Background
A conventional voltmeter measures voltage at a given point in a circuit using a high-impedance input. The goal with such a voltmeter is to determine the voltage present at the measured contact point without imposing any significant load on the point. A high-impedance input (relative to the impedance of the point contact) ensures that the voltmeter measures the point contact without changing the current flow within the measured circuit.
In certain applications however it is important to be able to measure a circuit's ability to deliver a minimum or threshold voltage under a specified constant or variable load that provides an indication of the health of the circuit. While such diagnostic tools may exist in certain fields, such tools are often expensive or difficult to use. One example of such a diagnostic tool is a curve tracer for semiconductor (active) and passive devices. A curve tracer can load an active or passive device with a varying voltage to induce a range of currents in such devices and display a graph of voltage versus current that provides the characteristics of the measured device. The use of a curve tracer requires some training and user expertise in the types of devices being measured. Curve tracers are relatively expensive diagnostic tools, and are generally designed for laboratory use only. Therefore, a need exists for improvement in the field of diagnostic tools that are inexpensive and require no user training, expertise or special training.
In the case of automobile electrical systems, it is increasingly common to find control units and actuators that are switched on and off with pulse-type signals rather than direct connections to a switch that provides a driving current. It is difficult to diagnose a fault in such a system because although a suitable voltage is typically present when the system is energized, it is not possible to determine whether a suitable current can be delivered to the control unit or actuator under an actual load, or whether a required minimum operating voltage will be present under a worst case current load condition. A digital voltmeter cannot be used in such an instance because such a device cannot respond as quickly as the circuit is switched, and may therefore indicate suitable average voltage while the necessary minimum voltage is not present under a momentary pulse-type switching condition. The diagnostic tool of the present invention can initiate a pulse-type load and measure the voltage level during the pulse-type load, as well as the available current capability of the measured circuit when the measured voltage drops to a pre-determined threshold voltage.
3. Description of Related Art
There are numerous types of diagnostic tools used to measure voltages in circuits including voltmeters, oscilloscopes, curve tracers, logic analyzers, and the like. A simple voltmeter allows a user to select a voltage range and apply test leads to the circuit to be measured. Voltmeters are high-impedance devices intended to measure a point contact without loading the measured circuit, since circuit loading can cause the circuit characteristics to change and thereby yield an incorrect voltage reading. A voltmeter is relatively simple to use, but does require the user to apply the test leads to the point contact as well as a reference point (often a ground or zero voltage reference, but also a secondary point contact). All voltmeters can measure a constant voltage, and some can measure a time varying voltage such as 50/60 cycle alternating current. However, the measurement of a time varying voltage requires that the voltmeter be optimized for a known time varying voltage such as those present in ordinary household sinusoidal alternating current circuits. An oscilloscope allows a user to measure a constant or time varying voltage and display the result in a graphic format. Oscilloscopes also use high impedance inputs in order to measure circuit voltages without loading the measured circuit. A curve tracer can be used to measure the electrical characteristics of a device by varying an input voltage and then displaying the result graphically.
While each of the above diagnostic tools are useful in varying instances, the cost of the device, limited diagnostic ability, and need for varying degrees of user skill limits the usefulness of such tools in certain applications.
The most cost-effective diagnostic tool for measuring a voltage at a point contact is generally a voltmeter. Prior art voltmeters are effective in the instance where a voltage measurement without load is desired. However, if a user desires to measure a point contact under a varying load, or needs to determine the load under which the measured voltage drops below a predetermined minimum value, a more sophisticated diagnostic tool is required. However, there currently exists no diagnostic tool that combines the ease of use of a conventional voltmeter with the ability to automatically measure a voltage under a varying load and determine the load at which the point contact measurement crosses a minimum value.
In one application of a diagnostic tool of the present invention, an automotive mechanic can use the diagnostic tool to measure a point contact voltage and receive an audible or visual indication from the diagnostic tool that the desired minimum voltage is present upon the automatic application of a predetermined minimum circuit load.
The diagnostic tool of the present invention provides for a small, relatively inexpensive device that can be used by an unskilled user to determine whether a point contact voltage can be delivered through the measured circuit under a given load. An objective of the present invention therefore is to be able to automatically measure such a point contact voltage and determine the load at which the voltage drops below a specific or predetermined value.